A. Field of the Invention
This invention relates generally to the field of aerial reconnaissance photography and camera systems used for such photography. More particularly, the invention relates to a method of compensating for forward motion of a reconnaissance vehicle while generating a frame of imagery of the scene of interest thereby preserving image resolution.
B. Description of Related Art
Long Range Oblique Photography (LOROP) cameras have been developed as a result of the need to obtain clear, high resolution pictures from longer ranges, typically from 10 to 50 nautical miles from the terrain of interest. The advent of LOROP cameras was an outgrowth of development of weapons technology, which could engage reconnaissance aircraft at ever-increasing distances, and geopolitical boundaries that became more and more difficult to encroach upon.
With the advent of LOROP cameras came the operational intricacies of using very sensitive and high performance instruments in a fashion that yielded the intelligence, i.e., image resolution, required of them. These operational issues were hostage to the technological limitations of the day. Initially, all cameras were film. Film LOROP cameras have been operated both as panoramic scanning (line scan) and framing cameras. Panoramic scan cameras collect an image with a smooth rolling motion of the camera while exposing film by pulling it passed a slit. The advantage of this approach was ease of implementation of the scanning mechanism. The disadvantage is that each line of exposed imagery was taken from a different perspective, hence the scanning system inherently was prone to creating geometrically and geo-spatially distorted images.
Subsequently, LOROP film framing cameras were employed. These cameras captured a frame of imagery by rapidly moving a slit across the film for exposure. The cameras utilized a scan head mirror assembly that could be moved in order to take successive frames of imagery at a selected depression angle relative to the horizon, depending on the target location.
Later, electro-optical line scan cameras entered the market as a filmless solution. Instead of film, the cameras used a solid state linear line scan charge coupled device (CCD) as a detector. These cameras used a scan mirror or the motion of the ground below the aircraft to scan the image across the line of photosensitive detectors that made up the CCD to form a frame, line by line. Again, the disadvantage of this method was that imagery was obtained from a different perspective as the aircraft moved, resulting in geometrically and geospatially distorted images.
Step framing cameras were developed which take a full frame of imagery at one time, then step the camera to a new angular position, take the next frame of imagery (with some overlap between the images to insure 100% coverage), step and generate a new frame of imagery, and so on until the desired scene is covered. The disadvantage of step framing cameras was that the stepping action was very difficult to accomplish with the whole camera, therefore it had to be broken into a scan head that performed the stepping and an image de-rotation mechanism, both of which were tied together by a synchronized drive system. The advantages of step frame cameras as compared to line scanning cameras are higher geometric fidelity and geo-spatial accuracy. Originally, full framing cameras were all film.
The next revolutionary step in the art of LOROP and tactical aerial reconnaissance cameras was the development of two-dimensional area array electro-optical (E-O) detectors. This occurred several years after the electro-optical linear arrays were first developed, and required semiconductor processing technology to mature many more years before such arrays were practical for reconnaissance use. Recon/Optical, Inc., the assignee of the present invention, in the early 1990""s, introduced large area focal plane arrays to the reconnaissance industry. One such array is described in U.S. Pat. No. 5,155,597 to Andre G. Lareau et al., the contents of which are incorporated by reference herein. Such cameras were the first large area arrays to be used in tactical aircraft, as well as strategic reconnaissance aircraft such as the high altitude SR-71 aircraft. These large area arrays had the advantage of providing an image from a single point in space giving excellent geometric fidelity. Moreover, the high pixel count, and optimal pixel size, allowed such cameras to produce imagery having outstanding image resolution.
Furthermore, as described in the ""597 Lareau et al. patent, it was possible to perform forward motion compensation in side oblique, forward oblique and nadir camera orientations electronically. U.S. Pat. No. 5,668,593, also to Lareau et al., describes a step-frame electro-optic camera system with electronic forward motion compensation. U.S. Pat. No. 5,798,786, also to Lareau et al., describes a method for compensation for roll, pitch or yaw motions of an aerial reconnaissance vehicle, in addition to forward motion compensation, electronically in the focal plane of an E-O detector. The ""593 and ""786 Lareau et al. patents are incorporated by reference herein.
Framing E-O LOROP camera systems were a logical platform to host the advanced detectors such as described in the Lareau et al. ""597 patent. Electro-optical detectors, such as described in the Lareau et al. ""597 patent, are capable of being fabricated from selected materials that can detect incident radiation in a variety of portions of the electromagnetic spectrum, and not just the visible spectrum. In particular, the advantages of large area framing can be enhanced by providing imaging capability in the infrared (IR) portion of the spectrum. A camera that generates frames of imagery in two distinct portions of the electromagnetic spectrum simultaneously is referred to herein as a xe2x80x9cdual band framing camera.xe2x80x9d The patent to Gilbert W. Willey, U.S. Pat. No. 5,841,574, also assigned to Recon/Optical, Inc., describes a multi-spectral, decentered aperture, catadioptric optical system particularly suitable for a dual band line scanning camera system having two linear electro-optical detectors, one for the visible or near IR (xcex=0.5 to about 1.0 microns), and one for either the mid-wavelength IR (xcex=about 3.0 to about 5.0 microns) or the long-wavelength IR (xcex=about 8.0 to about 14.0 microns).
A method is provided for generating a frame of imagery of a scene of interest with a camera while compensating for forward motion of a reconnaissance aircraft. The camera includes a Cassegrain objective optical subassembly having a primary mirror, a secondary mirror fixedly mounted relative to the primary mirror, and a flat azimuth mirror positioned in the optical path between the primary mirror and the secondary mirror. The camera also includes at least one framing image recording medium, such as a two dimensional electro-optical detector. The Cassegrain optical subassembly and framing image recording medium are incorporated into a camera housing mounted to the vehicle, with the camera housing defining an axis.
The method comprises the steps of:
(1) orienting, e.g., installing, the camera housing such that the camera housing is substantially parallel to the roll axis of the aircraft;
(2) rotating the primary mirror and secondary mirror about an axis orthogonal to the roll axis in the direction of flight of the aircraft, while maintaining the image recording medium in a fixed condition relative to the camera housing; and
(3) while rotating the primary mirror and secondary mirror as recited in step (2), rotating the azimuth mirror in the direction of flight at a rate one half the rate of rotation in step (2).
The azimuth mirror is rotated about an axis coincident with the axis about which the primary and secondary mirrors are rotated. In the illustrated embodiment, the azimuth mirror is located substantially at the center of the primary mirror.
In the illustrated embodiment, the method is performed in a camera implementing a roll-framing technique for generating sequential images of a terrain of interest. In this technique, the camera housing, including the objective lens subassembly and the framing image recording media, is continuously rotated about a roll axis.
The invention can be practiced in conjunction with single band, dual band or hyperspectral imaging implementations. The illustrated embodiment is a dual band system in which the camera includes first and second electro-optical detectors and first and second optical paths for different portions of the electromagnetic spectrum. The detectors generating frames of imagery of the scene of interest in two different portions of the electromagnetic spectrum simultaneously.